On the minimum electron transport coefficients in tokamaks in the range of low collision frequencies |
| |
Authors: | V G Merezhkin |
| |
Institution: | (1) Institute of Nuclear Fusion, Russian Research Centre Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182, Russia |
| |
Abstract: | There are two close empirical scalings, namely, the T-11 and neo-Alcator ones, that provide correct estimates for the energy confinement time in tokamaks in ohmic heating regimes in the linear part of the dependence τ E (\(\bar n_e \)) in the range of low values of \(\bar n_e \) and 〈ν e * 〉 ≤ 1. The similar character of electron energy confinement in this range, which expands with increasing magnetic field B 0, has stimulated the search for dimensionless parameters and simple physical models that would explain the experimentally observed dependences χ e ~ 1/n e and τ Ee ~ \(\bar n_e \). In 1987, T. Okhawa showed that the experimental data were satisfactorily described by the formula χe⊥ = (c 2/ω pe 2 )ν e /qR, in deriving of which the random spatial leap along the radius r on the electron trajectory was assumed to be the same as that in the coefficient of the poloidal field diffusion, while the repetition rate of these leaps was assumed to be ν e /qR. In 2004, J. Callen took into account the decrease in the fraction of transient electrons with increasing toroidal ratio ? = r/R and corrected the coefficient c 2/ω pe 2 in Okhawa equation by the factor σ ‖ Sp /σ ‖ neo . If one takes into account this correction and assumes that the frequency of the stochastic process is equal to the reciprocal of the half-period of rotation of a trapped electron along its banana trajectory, then the resulting expression for χe⊥ will coincide with the T-11 scaling: χ e an ∞ ?1.75(T e /A i )0.5/(n e qR) at A i = 1. If the same stochastic process also involves ions, it may result in the opening of the orbit of a trapped ion at the distance ~(c/ω pe )(m i /m e )1/4. In this case, the calculated coefficient of electron and ion diffusion D is close to D an ≈ χ e an /2. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|